This invention relates to ultrasonic transducers, and more particularly to Sell-type electrostatic transducers for use with an ultrasonic ranging system for a camera.
Ultrasonic ranging systems for cameras are disclosed in U.S. Pat. No. 3,522,764, German Pat. No. 864,048 and IBM Technical Disclosure Bulletin, Volume 9, No. 7, December, 1966, pp. 744-745. In each of these systems, periodic bursts of ultrasonic energy are transmitted toward a subject to be photographed, and the subject reflects some of the energy back to the camera. Simultaneous characteristics of the transmitted and received signals are compared necessitating separate sending and receiving transducers; and a control signal representative of subject distances is produced. The control signal is used to drive the lens mount of the camera to a position functionally related to subject distance whereby the subject is brought into focus.
Copending patent application Ser. No. 729,392, filed Oct. 4, 1976, discloses a ranging system for focusing a camera in response to transmission of a single burst of ultrasonic energy, the frequency of the burst varying from 65 to 50 kHz. This arrangement allows ranging, focusing and shutter actuation to take place sequentially in a relatively short time, as compared to human reflex time, in response to manual depression of a shutter release button, for example. To be practical, the transducer of a single burst ranging system must have a high mechanical damping factor to insure rapid decay of vibrations after termination of the driving signal in preparation for receipt of an echo. In addition, the electromechanical coupling of the transducer to the medium must be strong since the transducer must transmit the burst as well as receive its echo. The closest object that can be detected depends upon the time required for the vibration of the transducer to decay after the driving signal terminates. If the camera ranging system is to focus on objects as close as 25 cm, the decay must be completed with 0.3 msec.
A transducer with requisite mechanical damping and electromechanical coupling is the so-called Sell-transducer originally developed by H. Sell in 1937. In such transducer, a thin (i.e., 5-10 .mu.m) plastic film, metallized on one surface to form an electrode, is stretched over a relatively massive metallic counter-electrode, hereinafter termed the backplate, with the non-conductive surface of the film in contact with the backplate. The metallized surface of the film separated by the insulating film from the backplate defines a capacitor such that when a dc voltage is applied across the electrodes of this capacitor, irregularities on the surface of the backplate set up localized concentrated electric fields in the film. When an ac signal is superimposed on the dc bias during a transmission mode of operation, the film is stressed and oscillatory formations develop causing an acoustic wavefront to be propagated from the diaphragm. During a receive mode, a variable acoustic pressure on the diaphragm moves the film, producing a variable voltage across the electrodes.
The surface characteristics of the backplate apparently determine the frequency range and sensitivity of the transducer. With a very smooth, highly polished surface, the frequency range extends to 500 kHz although the sensitivity is rather low. With a surface roughened by sandblasting, or provided with grooves, the sensitivity is higher, but the upper frequency limit is lower.
In an article by K. Geide entitled "Oscillation Characteristics of Electrostatic Transducers Using the Sell Principle" appearing in Acustica, Volume 10, 1960, pp. 295-303, it is disclosed that a grooved backplate will maximize radiation of acoustic energy at frequencies in the range 5 to 80 kHz at atmospheric pressure. Experiments on a Sell-type transducer have been reported by D. Anke in an article entitled "Air Transducer Using the Sell Principle for Frequencies from 50 kHz to 100 kHz" appearing in Acustica, Volume 30, 1974, pp. 30-39. In such experiments, grooved backplates were employed, the width of the grooves ranging from 0.25 mm to 0.5 mm.
While transducers constructed in accordance with the above will provide suitable operation for some applications, they are not well suited for portable, mass-produced articles, such as the camera of the above-noted patent application where a specific overall efficiency is desired, e.g., high output, minimum side lobes, suitable receiving characteristics, and uniformity of operation, all consistent with low-cost, mass production manufacturing.
It is, therefore, an object of the present invention to provide a sonic transducer having enhanced output characteristics.
Another object is to provide a sonic transducer of easily implemented, compact construction having high conformity and repeatability of operation.
A further object is to provide a sonic transducer for use in a sonic ranging system for a camera.
Still another object is to provide a Sell-type transducer having improved directional characteristics for both propagation and reception of sonic energy for use in a camera ranging system.